What is a Wind?
The air currents in the motion are called “wind”. The wind is an important agent of erosion, transportation and deposition. Its work is particularly seen in arid regions.
Although wind erosion is not restricted to arid and semi-arid regions, it does its most effective work in these areas. Wind does erosion in three ways: (i) Deflation, (ii) Abrasion, (iii) Attrition and (iv) Deposition.
Removal and lifting of loose material from the surface of rocks is called deflation. The removal and lifting of material from surface, lowers down the level of land surface. The most prone areas before the wind erosion are the desert and the arid plains. In many plains and deserts areas deflation produces hollows or basins with their bottoms at water table. Such basins containing some water and greenery called Oases.
(2). Abrasion: The dust storms are another main cause of the erosion. Dust storms erode the surface material by carrying the minute particles/ grains of the sand in suspension. They dash and collide against the exposed rock masses and cause erosion. This process in which sand grains are used as tools for eroding rocks, is called “Abrasion”.
(3). Attrition: The particles which travels along wind collide with one another for thousands of time during each journey. The mutual collision causes their further breakdown and converting them into finer particles. The conversion of large particles into finer particles as a result of mutual collision is called “Attrition”.
(4). Deposition: The fine particles travel hundreds of meters away from their origin due to the wind current and dust storms. Their journey continues with the continual series of wind currents and wind storm. Some of the particles are stopped by elevated surfaces, while some continue moving. The particles which rest and deposit at some place are called the process of deposition.
Erosional Features of wind:
The important features of wind erosion are polishing of rock faces and formation of ventifacts and pedestal rocks.
(1). Ventifacts: Wind armed with sand abrades rock particles near the ground surface. This effect is called “sand blasting”. When pebbles and boulders are subjected to sand blasting they develop flat sides and sharp edges. If these stones contain coarse crystals of unequal hardness, they become pitted. Such stones, which are polished, pitted and contain sharp edges are called “Ventifact”. These stones are faceted by erosion of their windward side.
(2). Pedestal Rocks:
The undercut vertical columns of rocks with a wider head/top and narrower filament or base are called “pedestal rocks”. When wind blows, the sand particles being heavy, travel near the surface and cause under-cutting of the rock faces.
(3). Wind Transport:
Turbulent winds can easily sweep small dust particles and carry them to great distances in suspension. Sands, however, are transported in a series of jumps or these merely roll along the ground. The process by which sand particles travel in a series of jumps is called “Saltation”. The greater part of the sand grains are transported very near the ground surface and they are seldom lifted more than a meter above the ground.
(4). Wind Deposits:
The wind deposits are commonly called the “eolian deposits”. The rock particles in the eolian deposits are generally well rounded and are sorted according to their size and weight. Wind deposits are of two types: (i) accumulations of sand, called “Sand Dunes”, and (ii) deposits of silt, called “loess”.
(1). Sand Dunes:
The wind generally deposits sand in mounds. These mounds are called “sand dunes”. The sand traveling as bed load in wind accumulates wherever it meets any obstruction, such as a boulder or a bush. As the accumulation of sand grows, it traps even more sand. In this manner dunes are created. Sand dunes have a gentle slope 5 to 15 degree on the windward side and a steeper slope 20 to 30 degree on the lee side. The height of sand dunes depends on the wind speed and the size of sand grains. Dune heights of 30 meters are not uncommon. The sand dunes migrate slowly in the direction of wind movement. In some cases they move as much as 20 meters/year. The migrating sand dunes may advance and cover farmland, rail roads, highways, and other valuable property. Their movement may be checked by planting vegetation. The sand dunes are of four types:
- Transverse Sand Dunes: Transverse Sand Dunes have their longer axis at right angles to the direction of wind. They are formed in areas with strong winds where more sand is available.
- Barchans: Barchans are cresent shaped dunes. The convex side of which faces the wind direction. The horns of the dunes point in the direction of wind flow. Barchans are generally formed in areas where wind is mono-directional. They occur in groups in areas of greatest sand supply. The height of crescent dune does not exceed 30 meters, and their wing to wind length is about 300 meters.
- Longitudinal Dunes: The dunes which are elongated in the wind direction, are called “longitudinal dunes”. These dunes usually develop in under strong winds in areas where small amount of sand is available. The longitudinal dunes may reach heights of 100 meters and may extend for about 90 km. In the Arab countries these dunes are called “saifs” because they appear similar to an Arab Sword.
- Complex Dunes: In areas where the direction of wind varies, “complex dunes” are formed. These types of dunes are of irregular shapes. Shape of transvers, barchans, and longitudinal dunes may be seen, in a single dune of complex nature.
The suspended load transported load transported by wind consists mainly of silt and dust particles. When it settles down, it forms a blanket deposit of silt, known as Loess. These deposits are typically non-stratified and have a grayish yellow colour. Loess is composed of many minerals including quartz, feldspar, hornblende and calcite. These materials are derived by wind from deserts or from flood plains of rivers. Deposits of loess are very fertile. Loess deposits in some parts of China approach a thickness of 300 meters or more.